Web2

Web 2.0

Twitter

Sign Up (email required, Free)
Twitter in the Classroom
100 ways to teach with Twitter

Facebook

Using Facebook in the classroom
Great site providing examples and how to of Facebook in the classroom
Facebook in Education facebook page

Edmodo

Sign up (no email required, Free)
What is Edmodo

Blogs

Wordpress
Introduction to Blogging
Ways to use blogs in your classroom

Google Docs

What is Google Docs
Google docs for educators